Induction motor



May 29, 1 928. 1,671,488

R. RUDENBERG INDUCTION MOTOR Filed May 11, 1917 2 Sheets-Sheet l %90 mi dav baazzg May 29, 1928 1,671,488 R. RUDENBERG INDUCTION MOTOR Filed May 11, 1917 2 Sheets-Sheet 2 3 M It i I ,5; H/ I ,w M v 1 M /6 H w l/M m //l m ,M// 8 m/ Y *3 Q m 4 76 a M Patented May 29, 1928.

UNITED STATES PATENT OFFICE.

BEINHOL'D m'innm'znno, 0F CHARLOTTENBURG, NEAR BERLIN, GERMANY, ASSIGNOR,

BY MESNE ASSIGNMENTS, TO wnsrnvenous E ELECTRIC & MANUFACTURING COM- .PANY, A CORPORATION OF PENNSYLVANIA.

INDUCTION MOTOR.

Application filed May 11, 1917,

Serial No. 168,030, and in Germany March 30,1916.

(GRANTED UNDER THE PROVISIONS OF THE ACT OF MARCH 3, 1921, 41 STAT. L, 1313.)

Myinvention relates to induction motors and it has particular relation to the secondary windings of such motors.

One object ofmy invention is to increase the starting torque of induction motors of the type using a permanently closed-circuited winding while retaining good constantspeed characteristics and low starting current.

A more particular object of my invention is to obtain a large variation of the resistance of the armature conductors employed in the secondary winding of induction motors, and the like, by increasing the eddy currents induced in the conductors and the resulting current displacement in the conductors. J The foregoing and other objects of my invention will be best understood from the accompanying drawings, wherein Figure 1 diagrammatically shows a threephase alternating-current induction 'motor' connected to the line;

Fig. 2 shows a curve diagram of the relation of the torque to the slip of the motor;

Fig. 3 shows resistance curves indicating the necessary variation of the rotor resistance for maintaining predetermined torque conditions;

Fig. 4 shows curves indicating possible variat-ionsbf the apparentjohmic resistance of the rotor depending upon the slip;

Fig. 5 shows curves indicating the increase o fthe conductor resistance through current displacement, when ordinary conductors of 'a rectangular cross-section are used at commercial frequencies;

Fig 6 shows curves indicating the increase of resistance by current displacement for different subdivisions of conductors in dependence upon the numerical height of the slots;

Fig. 7 shows a transverse section of a slot containing two conductors, the figure serving as a basis for the mathematical explanation of the principles underlyingthe invention;

Fig. 8 shows a cross-section of a composite conductor, partly of electrically and partly of magnetically good conducting material;

Fig. 9 shows a longitudinal section of a squirrel-cage rotor having composite conductors, such as are shown in Fig. 8; and

Fig. 10 is a transverse sectionof a slot on the line XX in Figure 9.

If it is desirable, for controllin g purposes of some sort, to have the motor develop its highest possible torque within the entire torque other than the maximum, the resistance may be chosen at a value different than that given by curve 1'. For instance, in employing a resistance which is varied according to the thin curves 1', or 1' (Fig. 3), the torque curves d, or d drawn in thin dotted lines, in Fig. 2 are obtained. In the vicinity of the synchronous speed it is not possible of course to reduce the total'resistance of the rotor circuit to zero because of the internal resistance 7', of the secondary winding which necessarily must be left in the circuit, and which produces the normal slip of the motor.

As is well known, this failure of the asynchronous motor to start with a considerable torque without external resistances in the rotor circuits forms a decided disadvantage, as it renders necessary the use of slip rings and of special exterior switching devices and resistances for the rotor circuit. In a limited number of cases the low starting torque, produced by starting a rotor without external resistance, according to the heavy-lined starting curve of Fig. 2, for instance, will suflice, but even then it becomes necessary in most cases to make the inner rotor resistance greater than is desirable for normal working conditions.

Many attempts have been made to dispense with the exterior starting resistance and the slip rings of the motor, either by disposing the resistances and the switches controlling them within the interior of the rotor, or by increasing the effective rotor resistance by connecting different parts of the Winding in opposition. It has further been proposed to provide the rotor with a plurality of windings, a winding having a low resistance for use under normal working conditions and another Winding having a far higher resistance designed to take care ofthe starting torque.

The solutions of the starting problem mentioned in the first instance do not give nearly the advantages, so far as simplicity is concerned, which are gained by the squirrel-cage motor, which latter is adequate for normal work and has the most favorable eificiency. The solutions mentioned in the second instance have shown the effect of materially impairing the normal working conditions of the motor in the small number of cases where these expedients have been actually employed in practice, since it has proven to be impossible to dispose the several windings in equally favorable positions on the periphery of the rotor and, as is well known, the quality of an asynchronous motor depends to a very large degree upon such disposition of the windings.

It has finally been proposed, in order to I simplify at least to some extent the starting of asynchronous motors, to vary the exterior starting resistances automatically instead of by hand, for instance by making use of the increase of resistance ofmetallic conductors on being gradually heated by the current passing through them, or by taking advantage of the skin effect raising resistance of the conductors at higher frequency. However these constructions do not admit the use of the simple squirrel-cage rotor.

According to my invention, the problem of controlling asynchronous motors at any desirable torque is solved by artificially increasing the ohmic resistance of the armature conductors, necessary for generating the normal torque, by means of current displace-' ment in the slots, whenever a considerable 4 deviation from the synchronous speed has occurred. By the term current displacement I mean the crowding of the current towards the outer portions of the conductor similar to the manner in which the so-called 'skin effect manifests itself. By such cur rent displacement the outer portions of the conductor have a large specific current density and thus the ohmic resistance of the conducton is increased. 7.

In a motor in accordance with my invention all slip rings are dispensed with as well as all exterior controlling resistances and contacts. It may be provided in the practically simplest manner'with a squirrel-cage rotor having plain end rings. At normal,

almost synchronous speed it has an exceed ingly low slip, not greater than the slip of slip ring or squirrel cage rotors used heretoaccording to my invention, therefore, the

most economical and safe construction is combined with the simplest mode of control in starting as well as in reversing, even in reversing to different speeds, provided the pole connections in the stator are changed.

In order to facilitate the understanding of my invention I have plotted in Fig. 4 of the drawings a sample of the variations of the ohmic alternating-current resistance of difierent slot conductors, for a particular size of motor, plotting the resistance-variations as a function of the secondary frequency or slip s. The rate of. the variations of the resistance caused by the current dis placement in the slot conductors may, depending upon the particular construction of the motor, approach very nearly the desired rate of change shown in Fig 3, the resistance at small slips remaining substantially constant within a wide range, whereas it increases very largely at great slips. Polyphase motors for modern requirements re-. garding overload capacity and normal slip, require a rotor resistance at starting which is a rather high multiple of the normal resistance, from five-fold, in very small motors to fifty-fold in very large motors: In mediuin-size motors a fifteen-fold increase of the resistanceusually is desirable.

In Fig. 5- of the drawings the figures for the increase in resistance by means of current displacement are given as obtained in a typical prior-art squirrel-cage rotor of medium size, provided with conductors of rectangular sectibn, this increase being already,

quency of 50 periods per second are used so as to create the highest possible current dis placement} The height of the bars is assumed to be 3 centimeters'fll in.), this being, to all appearances, the greatest height used up to the present in squirrel-cage rotors.

The values of the resistance 7' plotted "in Fig. 5 show-that even in this extreme case and ainder expressly favorable conditions the increase of resistance does not by far reach the value required in the service described above. Instead, for instance, of a fifteen fold resistance increase only a three fold resistance increase is obtained. These figures become still more unfavorable. if, as usual, the resistance of the rotor and rings is made very high. so that the increase'of stance, if the resistance of the end rings is twice that of the rotor conductors, the variability in the example here chosen will only reach the value shown in Figure 5 in dotted lines. Accordingly, as everybody skilled in the art knows, normal squirrel-cage rotors do not develop any considerable starting torque. In order to obtain it, I artificially increase the current displacement.

The simplest way of attaining this end is the use of conductor bars having a greater sectional height. In the armature described above for instance a sectional height of cop per bars of about 5 centimeters (2 in.) would produce a five-fold increase of resistance, by reason of current displacement. In small motors this would be quite sufficient, but in the rotors of such motors it is impossible to arrange slots having the necessary= depth to accommodate bars of such height.

For motors of larger size, the sectional height of the bars would have to be in creased quite enormously, for instance to 15 centimeters (5.75 inches) in order to produce a fifteen-fold increase in resistance. 'Such abnormal increase in conductor height ob- -viously renders the construction of the rotors 'diificult.

' In order to increase the displacement of current I prefer to dispose a plurality of conductor bars in each slot, one below the other. These bars however are not simply placed in parallel but are arranged in such a manner,

for instance by series connection, that the distribution of the currents over the single individual conductors which prevails at lower frequencies, remains unchanged also at high frequencies.

' Contrary to a far spread theory the alternating current resistance, in the case where 40 the current displacement is considerable, is not diminished at first by such a subdivision of the conductor bars, but is very materially increased. Only when the conductors are subdivided to a very high degree, a consider- 5 able diminution of the alternating current resistance is obtained, such a diminution ordinarily forming the reason for such subdivision. For instance with a total sectional height of 8 centimetres (5 in.) of the copper in the slot, such as can still be obtained in a large type of motors, the use of a single copper bar will result only in an eight fold increase of resistance, whereas if the most favorable number of bars (four) is employed, one on top of the other, nearly the twenty fold resistance at the moment of starting is obtained. Such an increase of resistance however is suflicient for almost any large size motor.

The eifect of the current-displacement,or

eddy currents, in increasing the resistance of a radially-subdivided rectangular conductor embedded in an armature slot. such as shown in Fig. 7, when traversed by alternating currents, may be expressed by the formula (1) wf cos hZH-cos 211 where W is the resistance of the conductor when traversed by alternating current having a frequency f cycles per second, IV is the resistance of the'conductor when traversed by direct current, m is the number of radial subdivisions and H is, what may be termed, numerical conductor height and where A is the ohmic resistance of a part conductor per unit'length. in a longitudinal direction. (taken in absolute units), and B is the magnetic reluctance of the space occupied by a unit length of a part con ductor in a direction transverse to the slot. The values of A and B may be expressed by where h is the height of part condu tor in centimeters, d is the thickness of the con the several subconductors.

m-1 sin hH-sin H 3 cos hH cos H Since the ratio of the conductor thickness (d) to slot width; (b) is approximately constant (for thininsulation 1), the numerical height of the conductor is proportional to the height of the part conductors and practically independent of the conductor thickness.

In order to determine the totalheight of the conductor. or the depth of the slot, required for a definite resistance increase. and to more clearly bring out the effectof the subdivision of the conductor, we now introduce the total height of the conductor or the depth of the slot' (6) t=mh and an analogous value, the numerical slot height T T he actual values of the conductor height h and the slot depth t are then determined from thenumerical height H or depth T, respectively and will'vary depending on the frequency of the current, and the constants P of the material filling the slot.

In Fig. 6 are plotted curves showing the e the numerical slot depths T, for different numbers of subdivisions m=1 tom-=8.

For instance, to obtain a fourteen-fold increase in resistance, a numerical slot depth T=14 must be used in case of a single conductor; a numerical slot dep'th-T=9 for a two-part conductor (m=2) a numerical slot depth T=6.7 for a three-part conductor,

dependence of the resistance ratio upon (m=3) a numerical slot depth T=6.9 for.

a four-part conductor, (922 4); and so forth. The numerical slot depth 6.7 is the minimum value for a fourteen-fold resistance increase.-

It thus appears from the curves that, for each desired value of. resistance increase from the standstill to the running condition, there exists a predetermined minimum numerical slot depth below which it is impossible to obtain the desired resistance increase through current displacement.

To each minimum value of the numerical or actual slot depth, there corresponds a particular number of subdivisions into which the total conductor height-must be divided in order to obtain the desired re- A The minimum numerical sistance increase. I slot "depth T is shown, as a function of the resistance'increase, by the dotted line which envelops the curves representing the dependence of the resistance increase upon the number of the subdivisions. I

Accordingly, with a motor designed for a definite operating frequency, there exists a definite minimum slot depth, or total conductor height, and a predetermined o timum number of radial subdivisions oft e conductor, which will produce the automatic resistance increaseof the secondary induction motor winding necessary to give favorable 1 starting and running conditions.

I have found that, independently of the desired resistance increase, the minimum slot depth is always obtained by makingithe numerical, or actual, height of the individual part-conductors of the same definite value and using oneor' more such conductors, one on the top of the other, depending upon the desired resistance increase. The particular value of the height of the part-conductors is obtained by dividing the minimum numerical slot depth T for any particular value of resistance increase, by the corresponding number of subdivisions m. It will be found that we obtain a sectional numerical conductor height H of about 2.1 centimeters,

.which remains the same for practically any value of resistance increase.

In accordance with my invention, the secondary member of the induction motor is rovided with "a winding designed to automatically give the desired resistance increase between the starting condition, when the current in the winding has the' full line frequency, and the running condition, when the slip-frequency of the current is very low, by making the total conductor'height and the number of subdivisions in the conductor such that the numerical slot depth is approximately equal to the: minimum corresponding to the required resistance increase by current displacement. The numerical height H of the part-conductorsis then approximately 2.1 centimeters. The value of the numerical conductor height 2.1 may be slightly departed from, and quite favorable proportions 'are obtained in practice when the numerical height of the art-conductor ranges between 2 and 2.5 centlmeters.

In order to obtain the above-described automatic adjustment of the resistance necessary to give favorable starting and running conditions in motors operating on commercial frequencies and using copper for the secondary Winding, the slot depth must be made considerably greater than in the ordinary motors used heretofore. If such motors are designed in accordance with the recognized principles of economy in the materials, there is obtained a rotor having wide teeth and low tooth saturation and narrow conductors of relatively great total height. The low tooth saturation results in decreased flux-pulsation losses and improves the efliciency of the motor. p

The part conductors, which are arranged in the slots of the rotor, one on the top of the other, are suitably connected to constitute closed-circuited windings'responsive to the field induced by the primary member, care being taken to secure substantially equal distribution of the current among the several subconductors. If the conductors are connected as a phase winding, part conductors disposed at diiferent heights in the several slots are connected in series so that the total electromotive forces induced in each closedcircuited part circuit are equal. In case of a squirrel-cage winding, the part conductors are transposed within each slot, as indicated in Fig. 9, so that the electromotive forces induced in each part conductor are equal.

In distinction from the use of transposed windings in the prior-art machines for diminishing the current displacement, the transposition isused in the motors constituting the subject matter of my invention for increasing the current displacement.

For commercial frequencies (about cycles) and non-magnetic copper conductors See.

the actual conductor height 71. and slot depth become approximately equal to the numerical conductor height H and slot depth T, respectively. as will be seen from equation (7), which becomes If a considerable resistance increase is desired, the required slot may thusbecome too deep for practical constructions. According to my invention, a relative'l high value of the numerical slot de th is o tained with relatively small actual s ot'depth by increasing the magnetic conductivity of the slot contents, an increase in the conductivity a increasing the factor with which the actual slot depth 25, in equation (7), must be multiplied to obtain the numerical slot depth.

The simplest way for increasing the magnetic conductivity of the contents of the slot is to make the conductors of iron. The electric conductivity of iron conductors is, however, so small that the motor has too high losses near synchronism and, accordingly, the preferred form of my invention employs conductors which are partly of a material of good electric conductivity and partly of a material of good magnetic conand u=l),

. ductivity. Such conductor is shown in Fig.

8, wherein layers of magnetic material 2) are disposed betwen the part-conductors w of good conducting material, the individual part-conductors being suitably connected into a closed circuit, as explained above. In this way, I am able to obtain. with an actual slot depth of 2 to 3 centimeters, a numerical slot depth of 8 to 12 centimeters, which is sufiicicnt to produce a 12 to 18 fold increase in resistance at commercial frequencies.

Composite conductors may also be used in connection with squirrel-cage rotors, such as shown in Figs. 9 and 10, wherein two part-conductors w of good conducting material are disposed one on the top of the other in the slots and transposed in themiddle so that each part-conductor is located for one half of its length inthe lower portion'of' the slot and for the other half of its length in the upper portion, in order to secure equal distribution of the current between the two part-conductors. Eachpartconductor is split in the middle and an iron tiller '2: is embedded between the two portions of each part-conductor to increase the magnetic conductivity of the slot in accordance with the explanations given hereinabove. The magnetic fillers 41 preferably assist in conducting the induced currents, tlius utilizing all of the materials filling the s ot.

I claim i 1. Inan asynchronous machine, a secondary member comprising a magnetic core having a plurality of peripherally disposed slots, a short-circuited induced winding forsaid member comprising a plurality of conductor bars arranged one on the top of the other in said slots and so connected into closed circuits that the induced current is substantially equally distributed between all of the bars of a'slot and magnetic layers dis-- posed between said bars.

2. An asynchronous motor designed tooperate with currents of a predetermined line frequency, comprising a primary member and i a secondary member, said secondary member comprising a cylindrical magnetic core having slots disposed near the periphery thereof, a permanently closed-circuited induced winding in said slots, said winding comprising a plurality of conductors radiallyv disposed in the slots one on the top of the other and so arranged that the induced currents are equally distributed between the individual conductors, magnetic material interposed bemember comprising a cylindrical magnetic core having slots disposed near the periphery thereof, a ermanently closed-circuited induced win ing in said slots, said winding comprising a plurality of conductors radi ally disposed in the slots one on the top of the other and so arranged that the induced currents are equally distributed between the individual conductors, magnetic material interposed between said individual conductors to increase the ma netic field across the slots, the numerical heig t of the individual consquirrel-cage winding comprising relativelyhigh conductors disposed in said slots and short-circuiting rings at the ends of the core,

. each conductor being radially subdivided into a plurality of part-conductors, said partconductors being transposed within the slots so that each part-conductor has induced therein approximately the same voltage, magnetic material interposed between said part conductors to increase the magnetic field across the slots, the numerical height of the individualpart-conductors with the associated magnetic material being approximately 2.1, and the total conductor height being suflicient to produce a material resistance increase of said squirrel-cage winding between the normal running and starting conditions.

ber having a plurality of relatively deep slots, a squirrel-cage winding comprising a pair of relatively dee conductor bars per slot, whereby said winding has relatively high reactance and relatively large eddy current losses when the secondary current is of relatively high frequency, said conductor bars being offsetat their center and arranged so that the top bar becomes the bottom bar and the bottom bar becomes the top bar, the conductor bars in the same slot being insu lated fromeach other and connected at their ends to common end rings, and layers of magnetic material embedded in said bars.

6. In an induction motor, a magnetic member having a plurality of relatively deep slots, a squirrel-cage winding having deep conductors positioned in said slots whereby said conductors have a high reactance and a large eddy current loss when the secondary current is of relatively high frequency, each conductor of said squirrel-cage winding be-.

ing subdivided into a plurality of bars vertically arranged are insulated from each other, means whereby the bars of the same conductor carry substantially the same current, and magnetic material interposed be? tween the individual bars.

7. In a dynamo-electric machine, a magnetic core having slots, a winding comprising a plurality of highly conductive bars disosed adjacent to each other in said slot,

means for causing the adjacent bars to carry equal currents, notwithstanding the differences in the amount of fiux interlinked with the individual bar, and magnetic material interposed between adjacent conducting bars.

8. In an electric machine, a magnetic core and a plural-strand win-ding on said core, the individual strands being transposed throughout the winding to obtain substantially equal flux linkage for each individual strand and magnetic material embedded between the strands.

9. In a short-circuited armature for asynchronous motors having a slotted iron core, at least one conductor bar in each slot composed of a .plurality of good conducting elements disposed one above the other to cause displacement of current and increase of the conductor resistance. at great slips, the numerical height of said conductor elements being greater than 2 and smaller than 2.5, and magnetically conducting layers embedded between said conductor elements to increase the slot leakage and thereby the current displacing property of said elements.

10. In a short-circuited armature for asynchronous motors having a slotted iron core, at least one conductor bar in each slot composed of a plurality of good conducting elements disposed one above the other to cause displacement of current and increase of the conductor resistance at great .slips, i 5. In an inductlon motor, a magnetic memand being transposed relatively to each other in the slot at least once throughout the length of the slot, the numerical height of said conductor elements being greater than 2 and smaller than 2.5, and magnetically conducting layers embedded between said conductor elements to increase the slot leakage and thereby the current displacing property of said elements.

11. An induction-motor secondary member comprising a magnetizable core having a plurality of peripherally disposed slots or perforations and a lurality of conductor bars of good electricafconductivity and magnetic bars or bridges of good magnetic conductivity disposed at different radial heights in said slots or erforations, said conductor bars, at least, being so connected into closed circuits that the induced'current is substantially the same in all of the bars, the number of superposed bars and the radial height of each bar being so chosen that the depth of said slots or perforations is substantially a minimum for any given ratio of the linefrequency or starting resistance to the slipfrequency -or running resistance of said motor secondary member. 12. An induction-motor secondary member comprising a ma netizable core having a plurality of perip ierally disposed slots or perforations and a short-circuited induced winding comprising a plurality of conductor bars disposed atdifl'erent radial. heights in said slots or perforations and so connected into closed circuits that the induced current is substantially the same in all of the bars, the number of superposed bars and the radial height of each bar being so chosen that the depth of said slots or' perforations is substantially a minimum for any given ratio of the line-frequency or start- 'ing'resistanoe to the slip-frequency or runterial disposed therebetween.

' I 13. In a squirrel-cage induction motor having a high starting torque and a low slip,

a squirrel-cage secondary winding comprising composite bars, each bar comprisingtwo spaced, transposed portions of a material having good electrical conductivity and a filler of magnetic material disposed therebetween, the spaced parallel portions being joined together at the ends.

14.111 a squirrel-cage induction motor having a high starting torque and a low slip,

a squirrel-cage secondary winding, each bar of said winding comprising a plurality of transposed bars; each of the transposed bars comprising two spaced, parallel portions of i a material having good electrical conductivity and a filler of magnetic material'disposed therebetween, the spaced parallel portions being joined together at the ends.

15. In a squirrel-cage induction motor having a high 'startin torque'and a low slip, a squlrrel-cage seco'n ary winding comprising composite bars, each bar comprising twospaced, parallel portions-of a material having good electrical conductivity and a filler of magnetic material, disposed therebetween, the spaced parallel portions being joined together at the ends, and the height, permeability and specific resistance of the composite bars being such that the; numerical height, as defined in the specification, is of the order of 2. y

16. In a squirrel-cage induction motor having a high startin torque and a low slip, a squirrel-cage secon ary windin each bar of said winding comprising a p urality of transposed bars, each of the transposed bars comprisin two spaced, parallel portions of a materia having good electrical conduc-' tivity and a filler of magnetic material disposed therebetween, the spaced parallel portions being joined together at the ends, and

the height, *Fpermeability and specific re- REINHOLD RUDENBERG. 

